One approach for obtaining second or higher harmonics has been to use a nonlinear element such as a diode which generates several harmonics and to filter out the undesired harmonics. Such an approach operates satisfactorily only if the input signal does not change appreciably in frequency, however. Otherwise, the frequency band of the various (including adjacent) harmonics of changing frequency overlap and cannot be separated by band pass filter.
Widely varying input frequencies of the input signal can sometimes be taken care of with a system using frequency conversion and single side band filtering, but such a system is too complex and expensive for many applications.
In accordance with this invention, the input signal of frequency f is applied to a gain controlled amplifier; this amplifier is disposed in a feedback loop which further includes a low pass filter and at least one non-linear element, such as a squarer or two input multipliers in the output of the gain controlled amplifier. The output of the non-linear element may be passed through a high pass (or band pass) filter to provide the output signal of harmonic frequency nf.